The general forms of most metal detectors that interrogate soils are either hand-held battery-powered units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held battery-powered units include detectors used to locate gold, explosive land mines or ordnance, coins, and treasure. Examples of conveyor-mounted units include gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to locate buried landmines.
These metal detectors usually consist of transmit electronics generating a repeating transmit signal cycle, of a fundamental period, that is applied to an inductor, for example a transmit coil, which transmits a resulting alternating magnetic field, sometimes referred to as a transmit magnetic field.
These metal detectors also contain receive electronics that processes a receive signal from a measured receive magnetic field to produce an indicator signal, the indicator output at least indicating the presence of at least one metal target within the influence of the transmit magnetic field.
Advanced metal detectors often include several operating modes that are selectable by the user. Through selection of one of these operating modes, the user can optimise the detector for operation with different types of ground, for finding different types of intended targets, and attenuating the effects of different types of EMI. For example, an operating mode might be suitable for use in an area having one type of ground and to locate a particular type of target (eg small gold nuggets), but less than optimal for detection of large gold nuggets in the same type of ground. For another example, an operating mode might be suitable for use in areas having magnetic ground but not as suitable for areas having saline ground. Such variability requires that, in order to optimise the detection or prospecting process, the user should attempt to determine the nature of the ground and decide the types of targets to be sought, then select the best operating mode, prior to detecting or prospecting.
In practice, selection of the best operating mode requires prior knowledge of the ground, the targets, and also experience of the user. Further, if the ground is not homogeneous and the type of the ground changes with position within a detection zone, it becomes inconvenient for a user to adjust for the best operating mode. Even if the user somehow knows the types of ground, it is inconvenient to continually change the operating mode to suit the continually changing ground.
There is thus a need for an improved method for selecting the best operating mode, each such mode normally involving a set of one or more corresponding functions for processing a receive signal.